To investigate the effects of environmental factors on breast cancer risk we have conducted a number of high-quality population-based studies. We have been working in the Sister Study cohort, a large prospective study of women who have a sister with breast cancer. The Sister Study is a study of over 50,000 women across the United States which has information on exposure to residential criteria air pollutants. We have also linked the Sister Study cohort with the Environmental Protection Agencys National Air Toxics Database (NATA). We evaluated the role of hazardous metallic air toxic pollutants in relation to breast cancer risk in the Sister Study cohort and demonstrated that living in areas of higher exposure to airborne toxic metals, in particular lead, cadmium and mercury, was associated with a higher risk of postmenopausal breast cancer (PMID:30198937). In this study, we used a statistical method called weighted quantile sum to identify an overall mixture effect of airborne metals and to identify the individual metals most strongly related to breast cancer risk. In addition, we found that these same airborne metals were also associated with an increased likelihood of having dense breasts, an important risk factor for breast cancer, in the Breast Cancer Surveillance Consortium, which is a nationwide population of women with mammographic data (PMID:30760301). We also considered airborne exposure to mammary carcinogens estimated by NATA using machine learning methods and found that airborne exposure to methylene chloride was related to a higher risk of breast cancer in the Sister Study (PMID:31226564). These same airborne mammary carcinogens were also modestly related to telomere length, a measurement of biologic aging, a manuscript describing these findings is currently in press. We have also done work to evaluate how breast cancer risk factors may be associated with DNA methylation-based measurements of biologic age (epigenetic age). We observed that women in the Sister Study who were long-term night shift workers had an accelerated epigenetic age (PMID:30879037), which is a predictor of increased risk of chronic disease and mortality. Similarly, we found that exposure to particulate matter among women in certain clusters with similar PM2.5 component profiles was also associated with accelerated epigenetic age (PMID:31387022). These PM2.5 component profiles also modify associations between PM2.5 and breast cancer risk, which is under review.